This invention relates to devices for mounting lenses, particularly ophthalmic lenses, in edge grinding machines of the type in which the lens is gripped between a pair of pads at the ends of coaxial rotatable shafts in a movable head of the machine, so that the edge of the lens is presented to a grinding wheel. Fixed to one of the shafts so as to rotate with it is a template of the desired shape of the lens. By cooperating with a stationary stop or anvil, the template limits the extent to which the shaft axis can approach the grinding wheel. As the lens is slowly rotated against the surface of the rapidly rotating grinding wheel, the edge of the lens is progressively ground away until the lens has the same shape as the template.
It is necessary that the lens be accurately positioned in relation to the template so that it is optically correct when fitted into the spectacle frame. Not only must the optical centre of the lens be correctly located relative to the shaft axis (usually but not always, the centre will lie on this axis) but also, in the case of a lens having a cylindrical component of surface curvature, the cylinder axis must be orientated at the ophthalmically prescribed angle in relation to the horizontal.
A usual procedure is to place the already surfaceground and polished lens in an instrument which enables the power (magnification) of the lens to be checked, the lens centred, and cylindrical axis angularly oriented in relation to the horizontal as prescribed. The lens is then marked with removable marks to indicate its optical centre and its correct horizontal axis. These marks enable the lens to be mounted in the edge grinding machine in the correct position, both as regards optical centre and axis orientation, in relation to the template.
Various devices are used for mounting the lens in the machine in the correct position, but all have certain disadvantages which the present invention avoids.